# [Lab]difference between diffraction grating and linear polarizer ?

1. Nov 4, 2012

### genxium

[Lab]difference between "diffraction grating" and "linear polarizer" ?

I'm recently looking for optical devices to do some experiments at home, like diffraction and polarization, but when I turn to Amazon for the goods, I found that both the sheets I want are marked: 500 lines/mm -- diffraction grating, and linear polarizing sheet(2nd picture of the product shows "500 lines/mm diffraction"). You can click the names to view the corresponding devices on Amazon. Note that this is absolutely not ads, I have to exemplify the dimensions. Maybe that "500 lines/mm" in the polarizing sheet link is not for the polarizer itself, I'm not sure.

This "500 lines/mm" confuses me, as the physics text book says, if I wanna observe diffraction, the dimension of the aperture should be comparable to the wave length of visible light(several hundreds of nanometers), thus apertures on the "diffraction grating" should suffice this criterion -- it's 2um/line = 2000 nm/line, and I think this is the interval of apertures, so "500 lines/mm" should be a good choice for diffraction grating.

However, the dimension requirement of polarizing sheet was not mentioned in my physics courses, I have no idea how large or small the apertures(or the lines) should be so as to allow polarization, is it also "comparable to the wave length of visible light" ?

If there's any flaws in my description, please point out and I'll correct them asap.

2. Nov 4, 2012

### Simon Bridge

Re: [Lab]difference between "diffraction grating" and "linear polarizer" ?

Um ...Polarizers are made a different way to diffraction gratings. It is not a matter of having lots of finely spaced lines ... which you will see if you cross two polarizers then slip a third, at an intermediate angle, between them.

The second pic (reproduced below) is a sketch with a polarizer depicted on the left and a diffraction grating depicted on the right, labelled "500 lines/mm linear diffraction", which may have confused you.

3. Nov 5, 2012

### genxium

Re: [Lab]difference between "diffraction grating" and "linear polarizer" ?

Simon, thanks for the quick reply !

I just checked the prevailing types of linear polarizers and yes they're totally different from gratings in principle.

However, I'm still confused about 1 thing. My impression on the "parallel lines of linear polarizer" actually came from an opencourse lecture video of MIT(Vibrations and Waves). Though it's been a long time and I can't remember in which lecture the Professor demonstrated this, but I wanna describe what I watched here first:

He used a transmitter to send polarized EM wave to a receivers(some tens of centimeters from the transmitter), if the receiver successfully receive the signal, it sounds.
Between the transmitter and receiver, there's a an aperture whose interval can be adjusted.
At first the aperture was large and the receiver sounded loud. The Professor slowly cramped the aperture and to a certain narrow scale, the receiver didn't sound anymore.

Thus I wondered if linear polarizer may have some similarities with gratings.

I'll find the lecture video, maybe there's something wrong with my memory, bcz according to the principle of polarization, that experiment should be making use of absorptive polarizer, but the phenomenon I remembered doesn't seem reasonable.

4. Nov 5, 2012

### ehild

Re: [Lab]difference between "diffraction grating" and "linear polarizer" ?

There are wire-grid polarisers, widely used in the infrared range. They consist of very thin parallel metal strips, so very similarly built as a diffraction grating.
http://en.wikipedia.org/wiki/Polarizer
They also diffract radiation, depending on the wire distance to the wavelength.

ehild

5. Nov 5, 2012

### genxium

Re: [Lab]difference between "diffraction grating" and "linear polarizer" ?

Thanks ehild ! Seems I have to learn polarization again, those I learned from textbooks are really insufficient T_T

6. Nov 5, 2012

### Simon Bridge

Re: [Lab]difference between "diffraction grating" and "linear polarizer" ?

Yah - the metal strips conduct part of the EM wave.
The phenomena are related, both diffraction and polarization involve the interaction of EM waves with a solid, but not the same. The unification of physical law is one of the fascinating aspects of the study of physics.

For your purposes, though, the density of the lines mentioned in the ad is not related to the polarizer.